The present invention relates to a method for manufacturing poly(2-hydroxyalkanoic acid), preferably a poly(lactic acid). The present invention also relates to a poly(2-hydroxyalkanoic acid), preferably a poly(lactic acid), obtainable by said method.
Poly(2-hydroxyalkanoic) acids constitute a class of polymer compounds which currently receive much interest. Examples of such polymers compounds are polyglycolide, poly(ε-caprolactone) and poly(hydroxybutyrate) as well as their copolymers. Nowadays, most attention however is devoted to polylactide, which is also referred to as polylactic acid and abbreviated as PLA. This polymer is an aliphatic polyester, which can be manufactured from renewable resources. This manufacturing involves the fermentation of starch or sugar into lactic acid. PLA is usually synthesized either by direct polycondensation of lactic acid (lactate monomers), or by ring-opening polymerization (ROP) of lactide (cyclic lactate dimers). High molecular weight polymers are usually produced by means of the second method using lactide. Such lactides are most often obtained by depolymerization of PLA oligomers as a result of ring-closures reactions in the presence of a suitable catalyst. After purification, the lactide can be polymerized into PLA of controlled molecular weight by means of a ring-opening polymerization reaction.
Several methods of manufacturing PLA by ROP of lactide, as mentioned in the opening paragraph, are known as such. In one example, WO2010/012770 (A1), a polylactic acid (PLA) was prepared by the melt polymerization of L-lactide in a continuous pilot-scale polymerization reactor. The process was carried out at a temperature between 100-240° C. which comprises the steps of: a) continuously providing cyclic ester monomer and polymerisation catalyst to a continuous mixing reactor, the reactor being operated at conditions effective for polymerisation to form a pre-polymerised reaction mixture, b) continuously removing pre-polymerised reaction mixture from the continuous mixing reactor and continuously providing pre-polymerised reaction mixture to a plug flow reactor, the plug flow reactor being operated under polymerisation conditions, wherein the reaction mixture is polymerised to a degree of polymerisation of at least 90%, to form polymer c) continuously removing polymer from the plug flow reactor. Residual lactide content in the PLLA was reported to be 4-5% if the plug flow reactor was operated at a temperature of approx. 190° C. However for commercial purposes it would be desirable to produce a PLA having much lower levels of residual lactide content, e.g. of less than at least 0.5%.
U.S. Pat. No. 5,770,682 discloses a method for producing a PLA, comprising the steps of carrying out a ring-opening polymerization of lactide to give polylactic acid, adding a compound capable of inactivating a catalyst for ring-opening polymerization of the lactide at the completion of the reaction, and removing unchanged lactide from the polylactic acid product by reducing pressure and/or allowing an inert gas to pass. US '682 discloses the use of phosphorus-based acids or their esters, aluminum compounds, or oxidizing agents as compounds capable of inactivating catalysts. However it is often undesirable to use free acidic compounds or compounds that may decompose to yield acidic species in high temperature polymerization production processes due to the risk of corrosion of the apparatus and also subsequent leaching of metallic species into the polymer. It is also generally desirable to limit the incorporation of metallic species into thermoplastics as they may limit the melt stability or cause degradation and/or discoloration upon subsequent melt fabrication into finished articles. Furthermore, the level of residual lactide in the PLA polymers was not reported in US '682 for the examples prepared by this method.
As another example, the patent document EP 2 271 696 B1 discloses a process for manufacturing PLA comprising the steps of combining lactide with a polymerisation catalyst, subjecting the mixture to polymerization conditions to form polylactide in the liquid state, adding an organic peroxide, subjecting the liquid polylactide to a devolatilisation step, and allowing the polylactide to solidify. EP '696 B1 discloses examples of using a peroxide, hydroperoxide, in stabilizing a polylactic acid (PLA) prepared by the melt polymerization of L-lactide in a similar continuous pilot-scale polymerization reactor. The melt stability of a PLA having 7% residual lactide was improved by the post-polymerization addition of hydroperoxide in the examples. However for commercial purposes it would be desirable to produce a PLA having much lower levels of residual lactide content, e.g. of less than at least 0.5%.
In Applicant's view, these known methods can be improved, particularly because it was found that the residual lactide level of the formed PLA in the known processes is not optimal. More precisely it has been shown that after the devolatilisation step an appreciable amount of lactide is reformed from the PLA in the liquid phase, via a process which is called ‘back-biting’. This process is disadvantageous with respect to the PLA end product. Firstly, it reduces the MW of the manufactured PLA after the devolatilisation step. Secondly, it increases also the lactide concentration in the ultimate PLA product, which increase is not desired because of the plasticizing effect of lactide as well as possible deposit of lactide on processing equipment.
In conclusion, it would be desirable to have a method to manufacture poly(2-hydroxyalkanoic acid), particularly PLA, by ROP in which the resulting PLA product contains a reduced amount of lactide and has improved melt stability against “back-biting”.